Application fields of porous silicon as a porous member include, e.g., the manufacture of an SOI (Silicon On Insulator or Semiconductor On Insulator) substrate. For example, Japanese Patent Laid-Open No. 5-21338 discloses a method of manufacturing an SOI substrate using porous silicon. In the SOI substrate manufacturing method disclosed in Japanese Patent Laid-Open No. 5-21338, a nonporous Si layer (a layer to eventually serve as an SOI layer) is formed on a porous Si layer, and the first substrate including the nonporous Si layer is bonded to the second substrate such that an insulator is located on the nonporous Si layer. Then, a portion from the back surface of the first substrate to porous silicon is removed from the bonded substrate stack, thereby obtaining an SOI substrate having the nonporous Si layer on a buried insulating layer.
Japanese Patent Laid-Open No. 5-21338 does not consider a porous structure in a region (e.g., a region from the surface to a depth of 100 nm) in the vicinity of the surface of the porous Si layer formed in the porous Si layer formation step.
Recently, demands have arisen for a thinner SOI layer as an active layer of an SOI substrate. The present inventors have found that the structure of a region in the vicinity of the surface of a porous Si layer to serve as an underlying layer for an SOI layer is extremely important in thinning the SOI layer. As will be described later in detail, the present inventors also have found that the surface layer of a substrate on which a porous Si layer is to be formed, having a hydrogen concentration higher than that of the remaining portion, affects the structure of the porous Si layer to be formed.
As prior-art references that pertain to adverse effects produced by diffusion of hydrogen in an Si substrate without a porous Si region, i.e., a general Si substrate and measures against them, there are available Japanese Patent Laid-Open Nos. 7-45573 and 5-21371. Japanese Patent Laid-Open Nos. 7-45573 and 5-21371, however, have nothing to do with application technology of porous silicon and, more particularly, a technique for processing a material substrate to form a thin high-quality nonporous layer on a porous Si layer.